Since the rare earth oxysulfide phosphor (in the form of Ln.sub.2 O.sub.2 S:Ln') has good emission efficiency, chemically stable, ease in handling, etc., Y.sub.2 O.sub.2 S:Eu is practically used as a red-emitting phosphor for color televisions and Gd.sub.2 O.sub.2 S:Tb is practically used as a green-emitting phosphor for projection cathode-ray tubes.
As a method for preparing this kind of phosphor, it has been known to employ a method wherein Ln.sub.2 O.sub.3 and Ln'.sub.2 O.sub.3 as rare earth oxide raw materials are dissolved in a mineral acid and then reacted with oxalic acid to coprecipitate as their oxalates, which are pyrolysed to form a Ln.sub.2 O.sub.3. Ln'.sub.2 O.sub.3 mixed crystal, and then a sulfiding agent and a flux are added to the mixed crystal, followed by baking, and a method wherein a sulfiding agent and a flux are added directly to Ln.sub.2 O.sub.3 and Ln'.sub.2 O.sub.3, followed by baking (U.S. Pat. No. 3,502,590). In these methods, the flux serves to let the above synthesis proceed easily and accelerate the growth of particles. Under these circumstances, in order to improve the formation efficiency of phosphors and accelerate the growth of particles, attempts have been made to add various kinds of phosphates as the flux, for example, YPO.sub.4 (Japanese Patent Publication No. 35555/1976), Na.sub.4 P.sub.2 O.sub.7 (Japanese Patent Publication No. 46182/1979), AlPO.sub.4 (Japanese Patent Publication No. 192484/1982), etc. On the other hand, with the trend to high quality color televisions and display tubes in recent years, improved quality fluorescent screens of cathode-ray tubes have been in great demand. To obtain a fluorescent film (dot, stripe) with an excellent packing property it is important that the dispersibility of the phosphor is good and that giant particles are not present. This is because that the presence of the giant particles is apt to cause defects such as pinholes. For fluorescent film properties, it is important to form three pure colors (dots or stripes) by coating. If a red-emitting phosphor particle is deposited on a blue or green picture element formed in the previous step and remains thereon, so-called color cross contamination which reduces color purity is caused. The color cross contamination is mainly caused by fine particles and thus it is desired to reduce the content of the fine particles. Namely, as the powder properties of phosphors, it is desired that the dispersibility is good, the contents of the large particles and fine particles are low and the particle size distribution is narrow.
It is also desired to obtain large particles (i.e. 7 .mu.m or more) which are not readily obtained by conventional synthetic methods, and thus it is required to provide a phosphor having a particle size the particular use of the phosphor.
However, conventional techniques for synthesizing phosphors are not adequate because there is a limit in the particle size control, i.e. by adjusting the baking temperature or the kind or the amount of the flux added. Particularly, the synthesis of large particle phosphors is difficult and, if the baking temperature is high, the synthetic conditions in a baking container become uneven which lead to the production of phosphors containing considerable amounts of the giant particles and fine particles and having a wide particle size distribution.